The W-CDMA and the OFCDM (Orthogonal Frequency and Code Division Multiplexing) scheme which is promising as a mobile communication technology in the fourth generation propose a three-step cell search method for reducing the time for cell search (for example, refer to Non-Patent Document 1). A wireless communication system of the OFCDM scheme using the three-step cell search method will be described below.
FIG. 1 shows an example of a conventional wireless communication system. In FIG. 1, mobile station 11 communicates with one of base station 12, base station 13 and base station 14. Base station 12 communicates with mobile stations in the area of cell 22. Similarly, base station 13 communicates with mobile stations in the area of cell 23, and base station 14 communicates with mobile stations in the area of cell 24.
In FIG. 1, when mobile station 11 whose location is registered in base station 12 moves and performs handover from cell 22 (own cell) to cell 23 or cell 24 (another cell), mobile station 11 reports a scrambling code that is an identification code specific to that cell, the frame timing of that cell, and other additional information, regarding another cell, to base station 12. This is referred as cell search for another cell. Here, another cell means a area where the mobile station can communicate with a base station apparatus to which the mobile station does not belong.
When cell search for another cell is performed in the OFCDM scheme, in the first step, the mobile station detects an FFT timing using correlation characteristics (GI correlation) between guard intervals (GI) and a valid symbol segment of a received signal, and obtains a symbol timing candidate.
After the FFT timing is detected, the mobile station needs to know the head of the frame in order to decode data. When CPICHs (Common Pilot Channel) are time-multiplexed before and after the frame as shown in FIG. 2, the mobile station can detect frame timings (frame boundaries) by learning the positions of adjacent CPICHs. Therefore, in the second step, the mobile station performs correlation calculation between adjacent symbols in the frequency axis direction based on the detected symbol timing candidate, and detects a frame timing based on the scale of the correlation value.
Next, in the third step, the mobile station obtains correlations between the CPICHs detected in the second step and all candidates for the scrambling code, and identifies the scrambling code based on the scale of the correlation value, that is, specifies the cell.
Next, the conventional three-step cell search will be described in detail. FIG. 3 is a flowchart of the conventional three-step cell search.
In FIG. 3, in step (hereinafter, “ST”) 31, FFT timing detection is performed, which is the first step of the cell search, and a symbol timing is detected.
Processing from ST33 to ST38 is performed on a plurality of FFT timing candidates k detected in ST31, from the symbol timing of k=0 (ST32). Further, processing from ST34 to ST36 is performed for each symbol, from i=0 (ST33).
In ST34, based on the symbol timing detected in ST31, correlation calculation is performed between adjacent symbols in the frequency axis direction. Then, it is confirmed whether or not correlation processing has been finished for all symbols in ST35. If correlation processing has been finished for all symbols, the flow proceeds to ST38, and, if not finished, i=i+1 is set in ST36, the flow proceeds to the next symbol, and correlation calculation between next adjacent symbols is performed again in ST34.
In ST38, it is confirmed whether or not correlation processing has been finished for all FFT timing candidates detected in ST32. If finished, the second step is regarded as being finished, the flow proceeds to ST39, and the third step processing is performed. If not finished, the second step continues, k=k+1 is set in ST37, the flow proceeds to the next FFT timing candidate, and the second step processing is performed again from ST33.
In ST39, it is determined whether or not the correlation values obtained through correlation calculation performed in ST34 are equal to or larger than a predetermined threshold. Then, the boundaries between adjacent symbols corresponding to the correlation values that are equal to or larger than the predetermined. threshold are used as frame timing candidates, and the frame timing candidates are rearranged in descending order of the correlation values.
In ST40, based on the frame timing candidates rearranged in ST39, correlation calculation is performed between all scrambling codes other than the scrambling code of the own cell and a received signal. Then, in ST41, it is determined whether the largest correlation value among the correlation values obtained in this correlation calculation is equal to or larger than a predetermined threshold. If the largest correlation value is less than the threshold, j=j+1 is set in ST42, and the correlation calculation of ST40 is performed again for the next frame timing candidate. On the other hand, if the largest correlation value is equal to or larger than the threshold, the scrambling code with this largest correlation value is identified as a scrambling code of another cell in ST43, and cell search is finished.
Non-Patent Document 1: Motohiro Tanno, Hiroyuki Atarashi, Kenichi Higuchi, and Mamoru Sawahashi, “Cell Search Time Performance of Three-Step Fast Cell Search Algorithm Employing Common Pilot Channel for Forward Link Broadband OFCDM Wireless Access”, Technical report of The Institute of Electronics, Information and Communication Engineers, NS2002-107, RCS2002-135, pages 99 to 104